Use TYPE_CHECKING instead of False

This allows for e.g. Jedi to infer types (it checks the name).

It was only used to support Python 3.5.0/3.5.1, where this is is not
available in the `typing` module.

Ref: https://github.com/davidhalter/jedi/issues/1472

Uses `TYPE_CHECKING = False` in `_pytest.outcomes` to avoid having to
work around circular import.

src/_pytest/python_api.py

@@ -23,9 +23,10 @@ from more_itertools.more import always_iterable
 import _pytest._code
 from _pytest.compat import overload
 from _pytest.compat import STRING_TYPES
+from _pytest.compat import TYPE_CHECKING
 from _pytest.outcomes import fail
 
-if False:  # TYPE_CHECKING
+if TYPE_CHECKING:
     from typing import Type  # noqa: F401 (used in type string)